Combine the sugar with 1 cup of the strawberries in a medium bowl. Using a fork, mash the berries to release the juices. Add the vin santo, lemon zest, vanilla and remaining 1 cup of strawberries. Stir together with a spoon. Divide the gelato among 4 glasses or bowls. Top with some of the strawberry mixture and a pinch of the flaky salt. Serve with a biscotti for dipping if desired.